1. Field of the Invention
This invention relates to a method of manufacturing a polarization-maintaining optical fiber, and particularly a polarizationmaintaining optical fiber which is suited for coherent communication.
2. Background Art
Generally, there are two kinds of polarization-maintaining optical fibers, namely a fiber including a circular core to which a stress is applied, such as an elliptical-jacket fiber, a PANDA fiber, a Bow-tie fiber, and another including a non-circular core, such as an elliptical core type and a rectangular-type core. In such polarization-maintaining optical fibers, the phase constant difference .DELTA.B between natural crossed polarized modes is widened in order to suppress cross talk of these modes to a minimum, so as to obtain the property of polarization maintenance.
Prior methods of manufacturing polarization-maintaining optical fibers whose cores are not circular in section are respectively shown in FIG. 1 (Japanese Patent Laid Open No. 24306/81) and in FIG. 2 (Japanese Patent Laid Open No. 92505/81) of the accompanying drawings.
According to the method of FIG. 1, first, a base material 113 which includes a circular core 111, circular cladding 112, and a silica glass tube 121, whose central portion is a circular cavity and whose section is oval, are fabricated (FIG. 1-a). Next, the base material 113 is inserted in the silica glass tube 121, forming a combined body, and such combined body is heated from the outside, forming a preform 130 (FIG. 1-b). Then, the preform 130 is heated and wire-drawn, so as to create an optical fiber including a fiber jacket 143 whose outermost profile of this section s approximately circular due to surface tension, the sections of both the core 141 and the cladding 142 being elliptical (FIG. 1-c).
In such a manufacturing method, since the silica glass tube 121 is employed, its diameter is limited to a certain degree. Accordingly, neither a preform of large diameter nor a long optical fiber can be obtained, so that it is not suitable for mass production. Also, since the core 141 is transformed from a circle to an oval through the process of heating and wire drawing, the configurations of the core 141 and the cladding 142 are apt to vary with fluctuations of heating temperature and the viscosity of the glass. Hence, it is difficult to fabricate, with high organic efficiency, an oval core optical fiber of pre-designed configuration, and rendering the fibers of the same configuration is difficult, as well.
In the method shown in FIG. 2, first, a vitreous tube 201 and a core member 203 having a rectangular section and shaped by mechanically grinding a round glass bar are prepared (FIG. 2-a). Next, there is formed a vitreous cladding layer 202 on the inner surface of the vitreous tube 201 by means of CVD (Chemical Vapor Deposition) (FIG. 2-b). Then, the core member 203 is inserted and fixed in the glass tube 201 which has the vitreous cladding layer 202 inside thereof, with the core member 203 and the glass tube 201 being aligned (FIG. 2-c). After that, the glass tube 201 and the core member 203 are rotated together and heated from outside, with both of them being shrunk, so that the core member 203 and the vitreous cladding layer 202 are blended and combined, creating a preform 205 (FIG. 2-d). The thusly formed preform 205 is wire-drawn to form an optical fiber which has a shape similar to the preform 205.
In this method, like the previous one shown in FIG. 1, since the glass tube 201 is utilized, a limitation is imposed on the diameter of the preform, and the high-volume production capability is low. Furthermore, during the process of fabricating the preform by shrinking the glass tube 201, the core member 203 and the glass tube 201 both easily change their shapes, so that it is difficult to manufacture the desired optical fiber at high yield.
Another related method of manufacturing a polarizationmaintaining optical fiber of the elliptical core type is disclosed in the publication of Japanese Patent Laid Open No. 130044/79.
Even if the cross talk of the polarization-maintaining optical fiber is small, that is to say, if it has an excellent extinction ratio of, for example, -30 dB/km, in an actual case the extinction ratio deteriorates to some -10 dB/km after a 100 km transmission, so that the modes whose cross talk becomes high function as an echo to the main signal and are transmitted to the receiver. Therefore, the transmission quality is deteriorated.
In order to prevent the deterioration in the transmission quality, it has been suggested that the aforesaid stress-applied fiber and the non-circular core fiber be combined together, so as to compensate the propagation velocity difference between orthogonally crossed polarized modes because of multi-refractive indexes generated by the applied stress. However, no suitable method of manufacturing thereof has heretofore been available.